1. Field of the Invention
This invention relates to metal-ceramic composite bodies produced by a metal infiltration process, e.g., silicon infiltrated composite bodies. More particularly, the invention relates to reaction-bonded and siliconized composite bodies having a boron carbide filler or reinforcement, or a reaction product of boron carbide, and to ballistic armor structures produced from such boron carbide or boron-containing composite bodies. The instant composite bodies are also extremely rigid, which in combination with their low specific gravity potential makes them attractive candidate materials for applications in precision equipment such as machines used to fabricate semiconductors. The instant invention also pertains to modifying the composition of boron carbide-containing composite bodies, to effect changes in properties of the resulting bodies, and/or in the processing parameters used to make the bodies.
2. Discussion of Related Art of Others
Silicon carbide (SiC) composites have been produced by reactive infiltration techniques for decades. In general, such a reactive infiltration process entails contacting molten silicon (Si) with a porous mass containing silicon carbide plus carbon in a vacuum or an inert atmosphere environment. A wetting condition is created, with the result that the molten silicon is pulled by capillary action into the mass, where it reacts with the carbon to form additional silicon carbide. This in-situ silicon carbide typically is interconnected. A dense body usually is desired, so the process typically occurs in the presence of excess silicon. The resulting composite body thus contains primarily silicon carbide, but also some unreacted silicon (which also is interconnected), and may be referred to in shorthand notation as Si/SiC. The process used to produce such composite bodies is interchangeably referred to as “reaction forming”, “reaction bonding”, “reactive infiltration” or “self bonding”.
Reaction bonded silicon carbide (sometimes referred to in shorthand notation as “RBSC”) ceramics combine the advantageous properties of high performance traditional ceramics, with the cost effectiveness of net shape processing. Reaction bonded silicon carbide ceramic offers extremely high levels of mechanical and thermal stability. It possesses high hardness, low density (similar to Al alloys) and very high stiffness (˜70% greater than steel). These properties lead to components that show little deflection under load, allow small distances to be precisely controlled with fast machine motion, and do not possess unwanted low frequency resonant vibrations. In addition, due to the high stiffness and hardness of the material, components can be ground and lapped to meet stringent flatness requirements. Moreover, as a result of very low coefficient of thermal expansion (CTE) and high thermal conductivity, RBSC components show little distortion or displacement with temperature changes, and are resistant to distortion if localized heating occurs. Furthermore, both Si and SiC possess refractory properties, which yields a composite with good performance in many high temperature and thermal shock applications. Finally, dense, high purity SiC coatings can be applied when extremely high purity and/or superior resistance to corrosion are required.
In many applications, including armor applications, weight is not a critical factor, and traditional materials such as steel can offer some level of protection from airborne threats such as ballistic projectiles and shell fragments. Steel armors offer the advantage of low cost and the fact that they also can serve as structural members of the equipment into which they are incorporated. In recent decades, certain hard ceramic materials have been developed for certain armor applications. These ceramic-based armors, such as alumina, boron carbide and silicon carbide provide the advantage of being lighter in mass than steel for the same ballistic stopping power. Thus, in applications in which having an armor having the lowest possible mass is important, such as (human) body armor and aircraft armor, low specific gravity armor materials are called for. The lower the density, the greater the thickness of armor that can be provided for the same areal density. In general, a thick armor material is more desirable than a thinner one because a greater volume of the armor material can be engaged in attempting to defeat the incoming projectile. Moreover, the impact of the projectile on a thicker armor plate results in less tensile stress on the face of the plate opposite that of the impact than that which would develop on the back face of a thinner armor plate. Thus, where brittle materials like ceramics are concerned, it is important to try to prevent brittle fracture due to excessive tensile stresses on the back face of the armor body; otherwise, the armor is too easily defeated. Rather, by preventing such tensile fracture, the kinetic energy of the projectile perhaps can be absorbed completely within the armor body, which energy absorption manifests itself as the creation of a very large new surface area of the armor material in the form of a multitude of fractures, e.g., shattering.
2.1 Sintered and Hot Pressed Ceramics for Armor Applications
Modern armor systems are required to provide protection against a wide range of projectiles (size, shape, hardness and impacting velocity) at minimal detriment to mobility of the soldier/vehicle (i.e., low weight and flexible). Such systems tend to contain ceramic tiles due to the high mass efficiency with which ceramics defeat projectiles. Until recently, the most common ceramics used within armor systems were sintered Al2O3, hot pressed SiC and hot pressed B4C. Typical properties of these materials are provided in Table 1.
TABLE 1Typical Properties of Commercial Sintered and Hot Pressed Armor Ceramics [1, 2]FractureTough-Hard-Young'sFlexuralnessnessDensityModulusStrength(MPa-Source(GPa)(g/cc)(GPa)(MPa)m1/2)SinteredCeramic 143.812753103.4Al2O3ProtectionCorporation(CPC)GradePTEX-300Hot PressedCeradyne233.204506344.3SiCGrade 146-3EHot PressedCeradyne322.504604102.5B4CGrade 546-3E
Owing to its low cost relative to SiC and B4C, sintered Al2O3 is often used in vehicle armor systems. However, due to its lower hardness and higher density, it is not suited to applications that have aggressive weight goals, such as personnel and aircraft armor systems. These systems tend to contain B4C or SiC.
Moreover, in many high performance applications, B4C is selected. Because of its very low density and very high hardness, it tends to provide the most weight-effective armor systems (particularly vs. light threats). The two primary drawbacks of hot pressed B4C are high cost and low fracture toughness.
In one of the earlier demonstrations of this technology, Popper (U.S. Pat. No. 3,275,722) produced a self-bonded silicon carbide body by infiltrating silicon into a porous mass of silicon carbide particulates and powdered graphite in vacuo at a temperature in the range of 1800 to 2300° C.
Taylor (U.S. Pat. No. 3,205,043) also produced dense silicon carbide bodies by reactively infiltrating silicon into a porous body containing silicon carbide and free carbon. Unlike Popper, Taylor first made a preform consisting essentially of granular silicon carbide, and then he introduced a controlled amount of carbon into the shaped mass. In one embodiment of his invention, Taylor added the carbon in the form of a carbonizable resin, and then heated the mass containing the silicon carbide and infiltrated resin to decompose (carbonize) the resin. The shaped mass was then heated to a temperature of at least 2000° C. in the presence of silicon to cause the silicon to enter the pores of the shaped mass and react with the introduced carbon to form silicon carbide.
U.S. Pat. No. 5,372,978 to Ezis discloses a projectile-resistant armor consisting predominantly of silicon carbide and made by a hot pressing technique. Up to about 3 percent by weight of aluminum nitride may be added as a densification aid. The finished product features a microstructure having an optimal grain size of less than about 7 microns. Fracture is intergranular, indicating energy-absorbing crack deflection. Moreover, the economics of manufacturing are enhanced because less expensive, less pure grades of silicon carbide can be used without compromising the structural integrity of the material.
U.S. Pat. No. 4,604,249 to Lihleich et al. discloses a composition particularly suited for armoring vehicles. The composition is a composite of silicon carbide and steel or steel alloy. Silicon and carbon particulates, optionally including silicon carbide particulates, are mixed with an organic binder and then molded to form a green body. The green body is then coked at a maximum temperature in the range of about 800° C. to about 1000° C. The temperature is then rapidly raised to the range of about 1400° C. to about 1600° C. under an inert atmosphere of at least one bar pressure. In this temperature range, the silicon and carbon react to form silicon carbide, thereby producing a porous body. The pores are then evacuated in a vacuum chamber, and the body is immersed in molten steel or steel alloy. The metal fills up the pores to produce a dense composite armor material.
In spite of the many outstanding properties, including high specific stiffness, low coefficient of thermal expansion, and high thermal conductivity enumerated above, reaction bonded SiC ceramics generally have low fracture toughness, and therefore may not be optimal in applications where impact loading will occur.
In response, materials investigators have experimented with various techniques for enhancing the toughness or impact resistance of such inherently brittle ceramic-rich materials. Perhaps the most popular approach has been to incorporate fibrous reinforcements and attempt to achieve crack deflection or fiber debonding and pull-out mechanisms during the crack propagation process.
Hillig and his colleagues at the General Electric Company, motivated in part by a desire to produce silicon carbide refractory structures having higher impact strength than those of the prior art, produced fibrous versions of Si/SiC composites, specifically by reactively infiltrating carbon fiber preforms. See, for example, U.S. Pat. No. 4,148,894.
More recently, German Patent Publication No. DE 197 11 831 to Gadow et al. disclosed a reaction-bonded silicon carbide composite body featuring high heat resistant fibers, in particular those based on silicon/carbon/boron/nitrogen, for example, carbon or silicon carbide. The composite body was formed by the melt infiltration of a silicon alloy into a porous preform containing the fibers. The alloying element for the silicon-based infiltrant may consist of iron, chromium, titanium, molybdenum, nickel and/or aluminum, with iron and chromium being preferred, and with 5-50% iron and 1-10% chromium being particularly preferred. The alloying addressed the problem of the jump-like internal strain caused by the volume increase of silicon upon freezing. Previously, in large or thick-walled articles, this cooling strain was sufficiently large in many cases as to manifest itself as microfractures throughout the composite body. Thus, the stability of the material was reduced, and a critical growth of the fractures was to be expected under application of alternating thermal and mechanical stress. Accordingly, by alloying the silicon phase, the jump-like strain was reduced or even avoided, thereby solving the problems associated with the silicon cooling strain. The exchange of some brittle silicon for a different metal also led to a clear increase in toughness and ductility of the composite body.
At a minimum, the matrix of Gadow et al. contains iron. In a further refinement, it is preferred to add to the iron-containing silicon matrix, further additives of chromium, titanium, aluminum, nickel or molybdenum in a suitable ratio for the formation of a passivation layer, so that it results in improved oxidation resistance and corrosion resistance. With specific regard to the aluminum addition, it is known from ferrous metallurgy that aluminum is never present in iron-based alloys in amounts more than about one or two percent. This is because aluminum is chemically reactive with iron, and additions of aluminum to iron will tend to form iron aluminides rather than result in elemental aluminum dissolved in iron.
In spite of the toughening afforded by the alloying, Gadow et al. still rely on fibrous reinforcement. In fact, they attribute part of the strength of the composite to its fibrous reinforcement, and the fact that they treated the fibers gently during the granulation process so as to not damage them and thus impair their strength. Fibers, particularly fibers based on silicon carbide, can be expensive. Further, short fibers such as chopped fibers or whiskers, can pose a health hazard, and efforts must be taken to insure that such fibers do not become airborne or breathed. Fibers are often added to a ceramic composition to enhance toughness through debonding and pull-out relative to the matrix. If another way could be found to toughen the silicon carbide composite bodies of interest, then one could dispense with the fibers.
Further, at least some of the infiltrant alloy compositions disclosed by Gadow, such as Fe35-Si65 alloy, have a melting point below that of pure silicon, and it would seem possible and even advantageous to take advantage of this phenomenon. Gadow acknowledges the lower melting point, but fails to take advantage of it, and instead recommends infiltrating at temperatures well above the silicon melting point, such as at 1550° C. and 1700° C., in his Examples 1 and 2, respectively.
Chiang et al. (U.S. Pat. No. 5,509,555) disclosed the production of composite bodies by a pressureless reactive infiltration. The preform to be infiltrated by the alloy can consist of carbon or can consist essentially of carbon combined with at least one other material such as a metal like Mo, W, or Nb; a carbide like SiC, TiC, or ZrC; a nitride like Si3N4, TiN or AlN; an oxide like ZrO2 or Al2O3; or an intermetallic compound like MoSi2 or WSi2, or mixtures thereof. In any event, the preform bulk density is rather low, about 0.20-0.96 g/cc. The liquid infiltrant included silicon and a metal such as aluminum, copper, zinc, nickel, cobalt, iron, manganese, chromium, titanium, silver, gold, platinum and mixtures thereof.
In a preferred embodiment of the Chiang et al. invention, the preform could be a porous carbon preform, the liquid infiltrant alloy could be a silicon-aluminum alloy containing in the range of from about 90 at % to about 40 at % silicon and in the range of from about 10 at % to about 60 at % aluminum and the carbon preform could be contacted with the silicon-aluminum alloy at a temperature in the range of from about 900° C. to about 1800° C. for a time sufficient so that at least some of the porous carbon reacted to form silicon carbide. Upon cooling, the dense composite formed thereby can be characterized by a phase assemblage comprising silicon carbide and at least one phase such as silicon-aluminum alloy, a mixture of silicon and aluminum, substantially pure aluminum or mixtures thereof.
One problem with infiltrating multi-constituent liquids into preforms containing large fractions of carbon is that the infiltrant chemistry can change dramatically over the course of infiltration, as well as from one location to another within the preform. Table 3 of Chiang et al. demonstrates this point. There, the infiltrant started out as being about 54 at % Si, 46 at % Cu, but after infiltration into a carbon preform, it was substantially 100% Cu. Such drastic compositional changes can make processing difficult; this same Table revealed that when the infiltrant alloy started out at about 30 at % Si, 70 at % Cu, pressure was required to achieve infiltration. Pressure infiltrations require much more complex and expensive equipment than do pressureless infiltration techniques, and usually are more limited in the size and shape of the parts that can be produced thereby. Thus, while the present invention is not limited to pressureless systems, unless otherwise noted, the infiltrations of the present invention refer to those not requiring the application of pressure.
Chiang et al. stated that their method allows production of composites very near net-shape without a need for additional machining steps. They described a number of non-machining techniques for removing the residual, unreacted liquid infiltrant alloy remaining on the reacted preform surface. Specifically, Chiang et al. stated that following infiltration, the composite body could be heated to a temperature sufficient to vaporize or volatilize the excess liquid alloy on the surface. Alternatively, the reacted preform could be immersed in an etchant in which the excess unreacted liquid infiltrant is dissolved while the reacted preform is left intact. Still further, the reacted preform could be contacted with a powder that is chemically reactive with the unreacted liquid infiltrant alloy such as carbon, or a metal like Ti, Zr, Mo or W.
In U.S. Pat. No. 5,205,970, Milivoj Brun et al. also was concerned with removing excess infiltrant following production of silicon carbide bodies by an infiltration process. Specifically, Brun et al. contacted the reaction formed body with an infiltrant “wicking means” such as carbon felt. More generally, the wicking means could comprise porous bodies of infiltrant wettable materials that are solid at the temperature at which the infiltrant is molten. Preferably, the wicking means has capillaries that are at least as large or larger than the capillaries remaining in the reaction formed body. Thus, infiltrant in the reaction-formed body that was filling porosity remained in the reaction formed body instead of being drawn into the wicking means and leaving porosity in the reaction formed body. The infiltrant could be silicon or a silicon alloy containing a metal having a finite solubility in silicon, the metal being present up to its saturation point in silicon.
The “wicking means” solution of Brun et al. to the problem of removing excess adhered silicon, while perhaps effective, nevertheless requires the additional processing steps of contacting the formed composite body with the wicking means and re-heating to above the liquidus temperature. What is needed is a means for eliminating or at least minimizing the degree of residual infiltrant adhered to the formed composite body.
2.2 Reaction Bonded Ceramics for Personnel Armor Applications
Reaction bonded SiC (sometimes referred to as “RBSC”) was first developed in the 1960's [3-5]. Other terms for the process include ‘reaction sintered’ and ‘self bonded’ [6]. Conventionally, the process consists of Si infiltration (liquid or vapor) into preforms of SiC+carbon. During the infiltration step, the Si and carbon react to form SiC. Typically, all carbon is consumed, yielding a product of porous SiC (vapor infiltration) or dense Si/SiC (liquid infiltration). The maximum SiC particle size used in the production of such bodies is generally in excess of a few hundred microns [3,4].
U.S. Pat. No. 3,725,015 to Weaver discloses composite refractory articles that, among other applications, have utility as an armor material for protection against penetration by ballistic projectiles. These compositions are prepared by cold pressing a mixture of a powdery refractory material (which could be boron carbide) and about 10 to 35 parts by volume of a carbon containing substance, such as an organic binder material or elemental carbon carbonaceous material to form a preform, heat-treating the preform to convert the carbonaceous material to carbon, and then contacting the heated preform with a molten metal bath, the bath containing at least two metals and maintained at a temperature between 1700° C. and 1900° C. The molten metal infiltrates the preform, the refractory material matrix sinters and at least one of the metallic constituents reacts with the carbon to produce a metal carbide. Because the thermal expansion coefficient of the metal mixture is close to or slightly greater than that of the refractory matrix, the composite shape cools to room temperature essentially free of cracks and residual stress. Weaver states that, while there are no rigid particle size parameters except those dictated by the properties desired in the final product, a maximum size of about 350 microns for the particles of the powdered materials that make up the mixture to be pressed is preferred. Further, he recommends adding to the metal mixture the same metal as the metal constituent of the refractory material. For example, he says that if boron carbide is the refractory material, the incorporation of about 6% of boron in the molten metal mixture prevents the dissolution of boron out of the boron carbide.
U.S. Pat. No. 4,104,062 to Weaver discloses a high density, aluminum-modified boron carbide composition that is well suited as protective armor against ballistic projectiles. About 70 to 97 percent by weight of boron carbide powder is blended with about 3 to about 30 percent of aluminum powder. A temporary binder is added to this mixture, and a preform is pressed. This preform is then hot pressed in an oxygen-free atmosphere at a pressure of at least 500 psi (3.5 MPa) at a temperature of from 1800° C. to about 2300° C.
U.S. Pat. No. 3,857,744 to Moss discloses a method for manufacturing composite articles comprising boron carbide. Specifically, a compact comprising a uniform mixture of boron carbide particulate and a temporary binder is cold pressed. Moss states that the size of the boron carbide particulate is not critical; that any size ranging from 600 grit to 120 grit may be used. The compact is heated to a temperature in the range of about 1450° C. to about 1550° C. where it is infiltrated by molten silicon. The silicon is not stated as containing any dissolved boron or carbon. The binder is removed in the early stages of the heating operation. The silicon impregnated boron carbide body may then be bonded to an organic resin backing material to produce an armor plate.
U.S. Pat. No. 3,859,399 to Bailey discloses infiltrating a compact comprising titanium diboride and boron carbide with molten silicon at a temperature of about 1475° C. The compact further comprises a temporary binder that, optionally, is carbonizable. Although the titanium diboride remains substantially unaffected, the molten silicon reacts with at least some of the boron carbide to produce some silicon carbide in situ. The flexural strength of the resulting composite body was relatively modest at about 140 MPa. A variety of applications are disclosed, including personnel, vehicular and aircraft armor.
U.S. Pat. No. 3,796,564 to Taylor et al. discloses a hard, dense carbide composite ceramic material particularly intended as ceramic armor. Granular boron carbide is mixed with a binder, shaped as a preform, and rigidized. Then the preform is thermally processed in an inert atmosphere with a controlled amount of molten silicon in a temperature range of about 1500° C. to about 2200° C., whereupon the molten silicon infiltrates the preform and reacts with some of the boron carbide. The formed body comprises boron carbide, silicon carbide and silicon. Taylor et al. state that such composite bodies may be quite suitable as armor for protection against low caliber, low velocity projectiles, even if they lack the optimum properties required for protection against high caliber, high velocity projectiles. Although they desire a certain amount of reaction of the boron carbide phase, they also recognize that excessive reaction often causes cracking of the body, and they accordingly recognize that excessive processing temperatures and excessively fine-grained boron carbide is harmful in this regard. At the same time, they also realize that excessively large-sized grains reduce strength and degrade ballistic performance.
A major advantage of the reaction bonding process is that the volume of the reaction-formed SiC is 2.3 times larger than the volume of the reacted carbon. Thus, by infiltrating Si into preforms that contain high carbon contents, ceramic bodies rich in SiC can be produced.
The reaction bonding process has several advantages relative to traditional ceramic processes (e.g., hot pressing, sintering). First and foremost, volume change during processing is very low (generally well less than 1%), which provides very good dimensional tolerance control and eliminates the need for final machining. In addition, the process requires relatively low process temperatures and no applied pressure, which reduces capital and operating costs. Moreover, fine high surface area powders capable of being densified are not required, which reduces raw material cost.
However, the vast majority of commercial reaction bonded SiC ceramics have coarse microstructures. This is due to the use of large SiC particles in the preforms and the fact that many of these materials are made using high levels of carbon in the preform. As the carbon reacts in an expansive manner with the Si to form SiC, the SiC particles in the preform are networked together to form large SiC clusters. Since the strength of a ceramic is controlled by the largest flaw within the stressed volume, a coarse grained material will tend to have low strength. Therefore, reaction bonded SiC ceramics are traditionally used for high temperature, creep, corrosion and wear sensitive applications, but not structural (strength critical) applications.
In the Third TACOM Armor Coordinating Conference in 1987, Viechnicki et al. reported on the ballistic testing of a RBSC material versus sintered and hot pressed silicon carbide materials. Not only was the RBSC substantially inferior to the other silicon carbides, Viechnicki et al. came to the general conclusion that purer, monolithic ceramics with minimal amounts of second phases and porosity have better ballistic performance than multiphase and composite ceramics. (D. J. Viechnicki, W. Blumenthal, M. Slavin, C. Tracy, and H. Skeele, “Armor Ceramics—1987,” Proc. Third TACOM Armor Coordinating Conference, Monterey, Calif. (U.S. Tank-Automotive Command, Warren, Mich., 1987) pp. 27-53).
Accordingly, in spite of the price advantage of RBSC relative to sintered or hot pressed silicon carbide, what the market has preferred has been a sintered or hot pressed monolithic ceramic product. In fact, according to some sources, RBSC had developed a reputation as not being worthy of serious consideration as an armor material.
The details of a ballistic impact event are complex. One widely held theory of defeating a ballistic projectile is that the armor should be capable of fracturing the projectile, and then erode it before it penetrates the armor. Thus, compressive strength and hardness of a candidate armor material should be important. The above-mentioned armor patent to Taylor et al., for example, suggests a correlation between strength and ballistic performance. They noted that when the size of the largest grains exceeded 300 microns, both modulus of rupture and ballistic performance deteriorated. Keeping the size of the boron carbide grains below about 300 microns in diameter permitted their reaction-bonded boron carbide bodies to attain moduli of rupture as high as 260 MPa, and they recommended that for armor applications the strength should be at least 200 MPa.
There seems to be a consensus in the armor development community that hardness is indeed important in a candidate armor material, and in particular, that the hardness of the armor should be at least as great as the hardness of the projectile. As for the strength parameter, however, those testing armor materials have had a difficult time correlating mechanical strength (both tensile and compressive) with ballistic performance. In fact, except for hardness, there seems to be no single static property that functions as a good predictor of good armor characteristics in ceramic materials. Instead, the guidance that has been provided from the armor developers to the materials developers based upon actual ballistic tests has been that candidate armors in general should possess a combination of high hardness, high elastic modulus, low Poisson's ratio and low porosity. (Viechnicki et al., p. 32-33)
As described in a recent paper [7], M Cubed Technologies, Inc. has optimized the reaction bonding process to allow relatively fine grained SiC and B4C ceramics with favorable mechanical and ballistic properties to be produced. Typical mechanical properties of the novel reaction bonded ceramics are provided in Table 2.
TABLE 2Typical Properties of Reaction Bonded SiC and B4C CeramicsFractureTough-Hard-Young'sFlexuralnessnessDensityModulusStrength(MPa-Source(GPa)(g/cc)(GPa)(MPa)m1/2)ReactionM Cubed223.063842843.9Bonded TechnologiesSiCGrade (Si/SiC)SSC-A3-82ReactionM Cubed282.573822785.0BondedTechnologiesB4C (Si/GradeSiC/B4C)RBBC-751
The property data in Table 2 clearly show some of the advantages of the reaction bonded ceramics, including high hardness and low density (especially for the B4C product). In addition, the reaction bonded B4C possesses an extremely high fracture toughness that is 2 times that of the hot pressed B4C (Table 1).
To date, the US Army and Marines have been supplied with hundreds of thousands of multi-curved ceramic tiles (both reaction bonded SiC and B4C) for use in SAPI (“small arms protective inserts”) products. The majority of present efforts are focused on the production of reaction bonded B4C tiles for use in E-SAPI (“enhanced SAPI”) plates. Against the E-SAPI threat (tool steel), reaction bonded B4C provides a good single shot V50 due to its high hardness, and demonstrates good multi-hit performance relative to hot pressed B4C due to its high toughness.
2.3 Issues with B4C and Si for Next Generation SAPI Ceramics:
Over the past 5 years, SAPI specifications have changed to meet the changing requirements in the field. Originally, the pacing threats were ball rounds (lead or soft steel). More recently, aggressive AP rounds (tool steel) have been added. In the future, it is quite possible that even more aggressive WC/Co-based AP rounds (e.g., M993) will appear on the battlefield. At this time, it will be necessary to have SAPI systems capable of providing weight and cost efficient armor protection for such threats.
The vast majority of ceramics currently being used in SAPI systems fall into two major categories, namely:
1. Hot Pressed B4C
2. Reaction Bonded B4C (composite of B4C, SiC and Si)
Issues exist with both of these materials for defeat of WC/Co AP ballistic threats. As is shown in Table 3, the pressure applied to a target by a WC/Co penetrator is far greater than that applied by a tool steel penetrator. Moreover, as shown in Table 4, B4C and Si undergo phase transformations when exposed to high pressure loads. Comparing data in the two tables clearly demonstrates WC/Co projectiles can apply pressures that will result in phase transformations of both B4C and Si. Such transformations cause volume changes that will result in damage to the solid material.
TABLE 3Pressures Applied to Targets by 7.62 mm Projectiles Constructed of Tool Steel and WC/Co [10]ProjectileHardness Pressure Applied byConstruc-ofProjectile During Projectile tionProjectile Muzzle Velocity TypeMaterial(kg/mm2)Impact (GPa)7.62 × 54 R mm B32Tool Steel 920 HV~157.62 × 51 mm NATO FFVWC/Co1550 HV~23
TABLE 4Threshold Pressures for Phase Transformations in Si and B4CPressure at Which PhaseMaterialTransformation Occurs (GPa)ReferenceSi~1611-12B4C~2013
Thus, new ceramic materials will be needed for future SAPI requirements. Moreover, the result of research and development activities aimed at producing such novel ceramics will lead to increased performance versus the current tool steel threats.
3. Discussion of Commonly Owned Patents
U.S. Pat. No. 6,503,572 to Waggoner et al., teaches that reaction-bonded or reaction-formed silicon carbide bodies may be formed using an infiltrant comprising silicon plus at least one metal, e.g., aluminum. Modifying the silicon phase in this way permits tailoring of the physical properties of the resulting composite, and other important processing phenomena result: Such silicon carbide composite materials are of interest in the precision equipment, robotics, tooling, armor, electronic packaging and thermal management, and semiconductor fabrication industries, among others. Specific articles of manufacture contemplated include semiconductor wafer handling devices, vacuum chucks, electrostatic chucks, air bearing housings or support frames, electronic packages and substrates, machine tool bridges and bases, mirror substrates, mirror stages and flat panel display setters.
U.S. Pat. No. 6,609,452 to McCormick et al. teaches that a fine-grained reaction-bonded composite material can provide excellent ballistic properties, particularly against small arms fire. By “fine-grained” what is meant is that no more than about 10 percent by volume of the morphological features making up the microstructure of the composite material should be permitted to be much above about 100 microns in size. The composite material preferably is highly loaded in one or more hard reinforcement substances, with silicon carbide being particularly preferred.
U.S. Pat. No. 6,862,970 to Aghajanian et al. teaches a composite body produced by a reactive infiltration process that possesses high mechanical strength, high hardness and high stiffness has applications in such diverse industries as precision equipment and ballistic armor. Specifically, the composite material features a boron carbide filler or reinforcement phase, and a silicon carbide matrix produced by the reactive infiltration of an infiltrant having a silicon component with a porous mass having a carbonaceous component. Potential deleterious reaction of the boron carbide with silicon during infiltration is suppressed by alloying or dissolving boron into the silicon prior to contact of the silicon infiltrant with the boron carbide.
WIPO Patent Publication No. WO 2005/079207 to Aghajanian et al. teaches that a boron carbide-containing preform that furthermore contains substantially no reactable carbon can be infiltrated with molten silicon or silicon alloy to form a composite body featuring boron carbide dispersed throughout a metal matrix containing silicon. Such a composite material may be referred to as “siliconized boron carbide”. This patent publication furthermore teaches that carbon alloyed or dissolved into the molten silicon prior to contact with the boron carbide of the preform may also help suppress chemical reaction of the boron carbide with the silicon.
The teachings of these commonly owned Patents and Patent Publications are incorporated herein by reference.